The invention relates to a process for the preparation of a polyurethane polymer, comprising the step of reaction of
A) polyester polyols with secondary hydroxyl end groups with
B) polyisocyanates which are chosen from the group comprising toluoylene-diisocyanate, diphenylmethane-diisocyanate, polymeric diphenylmethane-diisocyanate, xylylene-diisocyanate, naphthylene-diisocyanate, hexamethylene-diisocyanate, diisocyanatodicyclohexylmethane and/or isophorone-diisocyanate. The invention furthermore relates to polyurethane polymers prepared by such a process.
As a consequence of the α,ω-diols used for their build-up, polyester polyols which are industrially relevant for the preparation of polyurethane polymers contain primary hydroxyl end groups. The use of diols with completely or partially secondary hydroxyl end groups, such as, for example, 1,2-propylene glycol or dipropylene glycol, leads to polyester polyols which are approximately equipped, with respect to their end groups, like the diols from which they are built up. In the case of 1,2-propylene glycol, approximately 50% of the hydroxyl end groups would be secondary.
Diols which contain only secondary hydroxyl end groups, such as, for example, 2,3-butanediol, play no role on an industrial scale because of the amounts available on the market and the cost. An additional difficulty in the case of all diols containing secondary hydroxyl groups in polyester synthesis is that the rate of reaction with dicarboxylic acids is lower.
It is furthermore a particular disadvantage that as a consequence of the numerous short alkyl side groups, the properties of the polyurethanes prepared from such polyesters are significantly poorer than those of polyurethanes which are obtained from α,ω-diols. Conventional polyester polyols which are prepared with the diols mentioned with at least partially secondary hydroxyl end groups accordingly are both more expensive in production costs, in some cases more expensive in material costs, and also less suitable for the preparation of high quality polyurethanes. Polyester polyols with secondary hydroxyl end groups therefore have not hitherto had any relevant importance industrially, in contrast to polyether polyols.
It would be desirable to have available polyester polyols which contain α,ω-diol units within them and a unit with secondary hydroxyl end groups at their chain end. Such a structure would result in a reduced reactivity with respect to polyisocyanates and would make it possible, for example in the field of polyurethane flexible foams, also additionally to employ urethanization catalysts, such as tin salts, in addition to the amine catalysts, which chiefly drive the water reaction. In particular, as a result this opens up for the production of polyester polyurethane flexible foams the possibility widely used in the field of polyether polyurethane foams of better coordination of these two reactions with one another and of thereby obtaining, for example, processing advantages.
The functionalization of carboxyl groups in polyester polyol synthesis is disclosed in DE 36 13 875 A1. For the preparation of polyester polyols with an acid number of less than 1, a hydroxyl number of approximately 20 to approximately 400 and a functionality of expediently 2 to 3, polycarboxylic acids and/or anhydrides thereof and polyfunctional alcohols are subjected to a condensation reaction. This is advantageously effected in the absence of conventional esterification catalysts at temperatures of from 150° C. to 250° C. and optionally under reduced pressure. Polycondensation is carried out to an acid number of from 20 to 5 and the polycondensates obtained are then alkoxylated with 1 to 5 mol of alkylene oxide, for example 1,2-propylene oxide and/or preferably ethylene oxide, per carboxyl group in the presence of a tertiary amine. The tertiary amine is chosen from the group of N-methylimidazole, diazabicyclo[2,2,2]octane, diazabicyclo[5,4,0]undec-7-ene and pentamethyldiethylenetriamine. The catalyst is expediently employed in an amount of from 0.001 to 1.0% by weight, based on the weight of the polycondensate. The alkoxylation is advantageously carried out at temperatures of from 100° C. to 170° C. and under a pressure of from 1 to 10 bar.
In the process according to DE 36 13 875 A1, the esterification mixture is subjected to polycondensation to an acid number of from 20 to 5. It is stated as essential that the melt condensation is not interrupted too early. For example, if alkoxylation were to be carried out at an acid number of 25 or higher, the water content of the esterification mixture would be excessively high. This would result, however, in undesirable side reactions. If the synthesis of the polyesters is interrupted at an acid number of from 20 to 5, this means that a comparatively high content of terminal hydroxyl groups which originate from the alcohol component and are therefore as a rule primary is already present. The remaining carboxyl groups are then reacted with epoxides to shorten the synthesis time, terminal hydroxyl groups originating from the epoxides being obtained.
EP 0 010 805 A1 discloses a powder coating based on polyesters terminated by carboxyl groups, an epoxy compound and a choline compound of the formula [Y—CH2—CH2—N—(—CH3)3]+nXn-, in which X is OR or —O—C(O)—R and R is hydrogen or a C1-40 group and Xn- is an anion. Preferably, Y is OH or a group —O—C(O)—R. These powder coatings are less susceptible to yellowing and non-toxic. According to this specification, however, the epoxy compound contains on average two or more epoxy groups per molecule. The epoxy compound serves here to crosslink polyester molecules with one another and not to build up OH-terminated polyester molecules.
DE 28 49 549 A1 discloses a process for the preparation of polyether polyester polyols by reaction of a polyether polyol with a polycarboxylic acid anhydride to give an acid half-ester. The acid half-ester is then reacted with an alkylene oxide to give a product with an acid number of less than 5 mg of KOH/g. The reaction of the alkylene oxide with the acid half-ester is carried out in the presence of from 50 to 100 ppm, based on the starting polyether polyol, of a trialkylamine having 2 to 4 carbon atoms in the alkyl chain. The polyol obtained, however, is still based on polyethers and not on polyesters.
There is consequently still the need for alternative preparation processes for polyurethane polymers. In particular, there is a need for such processes using polyester polyols with secondary hydroxyl end groups prepared by alternative routes.